70 PROCEEDINGS OP THE AMERICAN ACADEMY 



soon assume the form of vessels themselves, and thus a rudimentary 

 bvmdle is formed. It happens rarely that two such scalax'iform ducts 

 appear simultaneously in parts of the prothallus remote from one an- 

 other. I only saw one such case, Fig. 13, a and b. 



It now becomes necessary to consider the relation of the scalariform 

 ducts to the other cells of the prothallus, and this must be done by 

 makinof longitudinal and transverse sections of the region in which the 

 ducts lie. We have such longitudinal sections in Figs. 2 and 7, of which 

 Figs. 1 and 6 give a view from above, but much less magnified. From 

 these sections we see that the prothallus forms a compact tissue in which 

 certain cells have assumed the character of scalariform ducts, while the 

 others remain unchanged. From no section made was I able to see any 

 trace of an archegonium. In two instances, when seen from above, a 

 combination of four cells led me to suppose that there was some signi- 

 fication to be attached to this arrangement. Fig. 5 is a magnified view 

 of Fig. 1, in which this arrangement in four occurs; but, as Fig. 4, a 

 longitudinal section, shows no connection between the four surface cells 

 and the scalariform ducts, I am compelled to regard the two cases 

 mentioned as having only accidentally such a superficial cell-confor- 

 mation. 



So far the changes mentioned have taken place in the plane of the 

 prothallus itself Now a change occurs which produces a growth in a 

 direction perpendicular to the prothallus, and this growth is easily dis- 

 tinguished from the usual embryo growth. A swelling is seen, generally 

 on the under surface of the prothallus, shortly after the appearance of 

 the scalariform duct. This swelling is situated on or very near the 

 line connecting the original duct and the nearest point of the concavity. 

 When there is a process, this swelling very often appears near its ex- 

 tremity, as in Fig. 6, b. When two such swellings appear simulta- 

 neously, they are generally situated side by side. In all cases, there is 

 seen back of the swelling the scalariform duct or ducts lying in the 

 substance of the j)rothalIus itself It is impossible for me to say in 

 which cells of the prothallus this swelling or outgrowth originates. 

 Longitudinal sections, as in Fig. 7, a, show no change by which the 

 cells of the outgrowing portion — which is, in this case, on the upper 

 instead of the lower surface of the prothallus, as is more commonly the 

 case — can be distinguished from the cells which are to remain a portion 

 of the prothallus. From the not unfrequent appearance of a bursting 

 through the surface, it can perhaps be inferred that the superficial 

 cells take no part in the growth. Certainly no particular cell or cells 

 seem to be the place of origin of the new growth, but it seems to be a 



